In this study, a thermo-plastic-martensite transformation coupled model based on the von Mises yield criterion and the associated plastic flow rule is developed to further improve the accuracy of numerical simulation during hot stamping. The constitutive model is implemented into the finite element program ABAQUS using user subroutine VUMAT. The martensite transformation, transformation-induced plasticity and volume expansion during the austenite-to-martensite transformation are included in the constitutive model. For this purpose, isothermal tensile tests are performed to obtain the flow stress, and non-isothermal tensile tests were carried out to validate the constitutive model. The non-isothermal tensile numerical simulation demonstrates that the thermo-plastic-martensite transformation coupled constitutive model provides a reasonable prediction of force-displacement curves upon loading, which is expected to be applied for modeling and simulation of hot stamping.

The Al-Si coating of ultra-high strength steel has been applied to hot stamping more and more widely, owing to solving the problem of oxidation and decarburization. However, the evolution of Al-Si coating during the heating process was rarely studied in the previous study. The tests about the influence of heating parameters, such as heating temperature, heating rates and dwell time, on properties of the Al-Si coating were carried out on the Gleeble-3500 thermal simulator. The properties of the Al-Si coating, for instance, volume fraction of FeAl intermetallics, α-Fe layer as well as porosity and 3D surface topography, were explored in the study. Results showed that more and more Kirkendall voids and cracks appeared in the Al-Si coating when the heating temperature exceeded 600°C. The heating rates almost had no influence on properties of the Al-Si coating when the temperature was equal to or lower than 500°C. The volume fraction of FeAl intermetallics in the coating with dwell time from 3 s to 8 min at 930°C was 0, 6.19%, 17.03% and 20.65%, separately. The volume fraction of the α-Fe layer in the coating changed from zero to 31.52% with the prolonged dwell time. The porosity of the coating ranged from 0.51% to 4.98% with the extension of dwell time. The unsmooth degree of the surface of the coating rose gradually with the increasing of heating rates and the extension of dwell time. The 3D surface topography of the coating was determined by the comprehensive effect of atoms diffusion, new formed phases, surface tension and the degree of oxidation of the coating surface. Experiments indicated that rapid heating was not suitable for the coating when the temperature exceeded 500°C. Experiments also demonstrated that enough dwell time was essential to obtain the superior properties of the coating.

Hot stamping technology of high strength steel creates new possibilities for vehicle manufacturers in promoting safety and fuel efficiency. The proportion of hot stamping parts in vehicles increases each year, and there are already hundreds of different types of hot stamping lines throughout the world. However, few studies regarding modeling of hot stamping process procedure have been reported, nor have there been studies regarding cooperative scheduling of manufacturing units (including heating units, transferring units, and forming units), or the relationship between productivity and energy consumption. With the shortening of the vehicle life cycle, auto parts manufacturing must be flexible for multiple types and small batches to meet the challenges of the market. In this paper, a model of the hot stamping process procedure based on finite state machine is established, then the model is adapted for the procedure control of a hot stamping line, consisting of multi-chamber furnaces, linear conveying robots, and a mechanical servo press. The cooperative scheduling of various manufacturing units, especially the method which matches multiple heating chambers with a single forming die, is focused on. Finally, a hot stamping line with multi-chamber furnaces is designed and implemented, and the analysis of its production sequence, energy consumption, and delivery cycles is performed. The results show that with the new modeling method of hot stamping process based on finite state machine, the new developed hot stamping line is capable of multiple production objectives and flexible configuration in accordance with production and delivery periods. This active configuration aids in extending equipment life and reduce energy consumption. The modeling method expands production models for hot stamping, from the mass production model to multiple types and small batches production model.

There has been a growing demand for safety parts with tailored properties in automobile industry. However, the understanding of tribological behavior of press hardening steels (PHS) on the tailored conditions is highly inadequate. The present work aims at creating new knowledge about the tribological characteristics of PHS on the tailored conditions and bridging this existing gap. The paper proposes an improved hot drawing tribo-simulator to simulate the realistic experimental conditions industry. Investigations were carried out on the condition of different initial heating temperatures, tool temperatures, austenitizing temperatures, cooling rates and microstructures. The presented results show that the whole frictional process is divided into three stages for both coated and uncoated steels. The frictional factor changes a lot and the peak value of frictional factor occurs for serious adhesive wear. The frictional factor rises as the tool temperature and austenitizing temperature rise. The surface morphology of tools indicates that the coating adhering to tool gets thicker as the tool temperature increases. With the increase of cooling rate, the frictional factor declines firstly and then rises to some extent. Flat dies with different temperatures are used to form specimens with different microstructures, which also affects the frictional factor and wear.

In traditional hot stamping process, heating of the sheet by radiation heating occupies most of cycle time, which limits the application of hot stamping in automotive industry. Thus a faster heating method has great significance on the hot stamping. The conduction heating overcomes shortage of the radiation heating because of higher heating rate and greater energy efficiency. It attracts increasing attention in the application of heating blanks in hot stamping. In the present study, a movable conduction heating device on die was designed in terms of the Joule’s Law. Heating experiments of boron alloyed steel were performed using the developed device. Heating rate and uniform temperature region were investigated in the non-heat preservation condition (NHPC) and the heat preservation condition (HPC). The results revealed that in the HPC, the heating rate was improved by 13.1 °C/s. In addition, the length of the uniform temperature region was lengthened by 15 mm. It was demonstrated that the HPC was preferred. Furthermore, it was indicated that the mechanical properties of the blanks in uniform temperature region of the conduction heating were also superior to that of the radiation heating.

In this study, austenitizing heat treatment before hot stamping of Al-10% Si coated boron steel is first investigated through environment scanning electron microscopy (ESEM) equipped with energy dispersive x-ray analysis (EDAX). The cracking behavior of the coating was evaluated using Gleeble 3500, a thermo-mechanical simulator under uniaxial plastic deformation at elevated temperatures. The extent and number of cracks developed in the coating were carefully assessed through an optical microscope. The coating layer under hot-dipped condition consists of an Al-Si eutectic matrix, Fe2Al7Si, Fe3Al2Si3 and Fe2Al5, from the coating surface to the steel substrate. The coating layer remains dense, continuous and smooth. During austenitization, the Al-rich Fe-Al intermetallics in the coating transform to more Fe-rich intermetallics, promoted by the Fe diffusion process. The coating finally shows the coexistence of two types of Fe-Al intermetallics, namely, FeAl2 and FeAl. Microcracks and Kirkendall voids occur in the coating layer and diffusion zone, respectively. The coating is heavily cracked and broken into segments during the hot tensile tests. Bare steel exposed between the separate segments of the coating is oxidized and covered with a thin FeOx layer. The appearance of the oxide decreases the adhesion of the Al-Si coating. It is found that the ductile FeAl is preferred as a coating microstructure instead of the brittle FeAl2. Therefore, the ductility of the Al-Si coating on hot stamping boron steel could be enhanced by controlling the ductile Fe-rich intermetallic phase transformations within it during austenitization. Experiments indicate that a higher austenitizing temperature or longer dwell time facilitate the Fe-rich intermetallics transformation, increasing the volume fraction of FeAl. This phase transformation also contributes to reducing the crack density and depth.

The variation in temperature of the blank would make major contribution to the mechanical properties of final component in hot stamping of ultra high strength steel (UHSS). It is important to use accurate thermal contact conductance (TCC) to carry out finite element simulation of hot stamping for reliable caculation results. In this paper, a flat compression test was performanced on a servo press. A fast response temperature measurement and data acquisition system was designed to obtain the temperature history of blank and die under different pressures. The thermal contact conductance between blank and die was got using an optimization algorithm. The error between the temperature curves using calculated thermal contact conductance and the curves of measurement temperature was analyzed. Result shows that reliable simulation results of temperature can be got through accurate thermal contact conductance.

In order to investigate the friction and wear behavior of high strength steel in hot stamping process, a hot strip drawing tribo-simulator is developed and the friction coefficient, which is an important parameter in the finite element modeling, is measured. The results have shown that the friction coefficient remains almost unchanged until temperature reaches 500 °C. It then increases sharply as temperature is increased from 500 to 600 °C. It has also been shown that the friction coefficient decreases as the drawing speed increases. The change in the dominate wear mechanism as the temperature and the drawing speed increases has been identified from SEM analyses of the worn surface. The dominate wear mechanism is the groove cutting at temperatures between room temperature and 500 °C, which changes to the adhesive wear at temperatures above 500 °C. The main wear mechanism is the adhesive wear at 25 mm/s, which changes to the slight groove cutting at 75 mm/s.

•The power spectrum density of magnetic Barkhausen noise was analyzed based on autoregressive model.•The low frequency signal components decrease as hardness increases, and increase as tensile stress increases.•The high frequency signal components increase as hardness increases, and decrease as tensile stress increases.•The magnetic Barkhausen signal was reconstructed by discrete Gabor expansion which removes unneeded signal components.An investigation was performed based on frequency domain analysis and reconstruction for the magnetic Barkhausen noise (MBN). The power spectrum density (PSD) of MBN was analyzed based on autoregressive (AR) model. The relations between PSD peaks and mechanical properties of high strength steel sheet were studied. The energy of low frequency signal components decreases as hardness increases, and increases as tensile stress increases. The energy of high frequency signal components increases as hardness increases, and decreases as tensile stress increases. After identification, the MBN signal was reconstructed by discrete Gabor expansion which remains desired signal components and removes undesired ones.